1. Field of the Invention
The present invention generally relates to write drivers for disk drives, and in particular to circuits for improving the driving voltage applied across write heads for write drivers in an "H" configuration.
2. Background Description
Driver circuits for writing data to magnetic disk drives have been evolving in response to various marketplace and related technological developments. One concern in the trend toward smaller computing equipment has been power dissipation. In order to reduce power consumption, write drivers were configured in an "H" topology, with the write head at the cross-bar of the "H". Switches at the four arms of the "H" (two "pull-up" switches above the cross-bar and two "pull-down" switches below form top and bottom halves, respectively; the two arms to the left of the cross-bar and the two arms to the right form left and right halves, respectively) are controlled by alternating pulses of a data signal so that at any one time only two arms of the "H" are conducting. One conducting arm is in the top half of the "H" and the other is in the bottom half; similarly, one conducting arm is in the left half of the "H" and the other is in the right half. When a data pulse is of one polarity, current flows from the top left arm through the cross-bar from left to right and down the bottom right arm. Similarly, when the data pulse is of the opposite polarity, current flows from the top right arm through the cross-bar from right to left and down the bottom left arm. All current which flows in the "H" configuration flows through the write head at the cross-bar, thereby minimizing power consumption, with the direction of the current through the write head depending on the polarity of the data pulse.
Since the speed bottleneck for a hard disk system is the write driver, it is desirable to have a means to improve the switching time of the write driver current. One technique for improving switching time is to use faster switches, as disclosed in U.S. Pat. No. 5,291,069 to Gooding et al. The Gooding patent describes circuitry for using bipolar transistors in an "H-driver" configuration, in place of field effect transistors.
Write drivers configured in the so called "H-driver" configuration have an inherent limitation to the voltage which can be applied across the differential output nodes. This limits the rate of change of current through an inductive load, such as a write head.
Schemes have been proposed to boost the effective driving voltage by coupling capacitive current into the output. For example, in U.S. Pat. No. 5,331,479 to Madsen there is described a circuit for boosting voltage across the recording write head of a magnetic disk drive during write current transitions. This increased voltage is created by switching a capacitor voltage so that the common reference voltage of the circuit is momentarily driven below ground. This circuit is a single circuit boosting the supply voltage during write transitions and allowing use of a lower voltage power supply. This approach transiently boosts current applied to the output during the transition.
Another approach which uses capacitive coupling to increase voltage to the write head is described in U.S. Pat. No. 4,647,988 to Takehara. However, this technique uses a center tap on the write driver load and pumps that center tap at data transitions which are sensed using a monostable multivibrator. Takehara does not disclose or suggest how write head voltage may be increased for a write driver which retains the power dissipation advantages of an "H" configuration.